1. Field of the Invention
This invention relates generally to the removal of vapor phase contaminants from a gas stream. More particularly, this invention relates to the removal of trace amounts of vapor phase air toxics, such as mercury, from the flue gas of a combustion process.
2. Description of the Related Art
The 1990 Clean Air Act Amendments, Title III, require major sources of air emissions to limit the discharge of certain chemical species. Certain of these chemical species are categorized as air toxics, and major sources are required to limit emissions to 10 tons per year for any given air toxin. Certain of these species may be present in the flue gas emitted from combustion processes. Therefore, cost-effective methods for controlling emissions of these species are of significant interest to the operators of these processes.
Air toxics and other species regulated by the 1990 Clean Air Act Amendments can be distributed in both the vapor phase and the solid phase in the flue gas from a combustion process. Typically, the air toxics are concentrated in the solid phase or particulate matter and can be effectively removed by the use of a particulate collection device, such as an electrostatic precipitator or fabric filter. Air toxics, such as mercury, that are present in the vapor phase are typically found in very low concentrations, for example, parts per million or less, making removal difficult.
Some techniques that are being evaluated for the removal of vapor phase species found at these low concentrations include the use of wet scrubbing systems or packed bed adsorption systems. Wet scrubbing systems are typically used to remove vapor phase species, such as sulfur dioxide that are present in higher concentrations than air toxics, such as mercury. Therefore, these systems may not provide the necessary removal efficiency for air toxics, such as mercury. Packed bed adsorption systems typically employ sorbents, such as activated carbon, for the removal of certain vapor phase species including mercury, but operation of such systems results in a high pressure drop and the necessity to regenerate or replace the sorbent material.
Other processes utilize injection of a fine powered sorbent material, such as activated carbon, into a flue gas to react with vapor phase species. The sorbents are then collected in a downstream particulate collection device, such as a fabric filter or an electrostatic precipitator. Moller, et al. (U.S. Pat. No. 4,889,698) discloses a process in which powdery activated carbon is injected immediately before, during or after an alkali reagent (limestone or sodium carbonate) spray dryer for simultaneous removal of acid gases and trace contaminants, such as mercury. The process requires the cooling of the flue gas by spray drying and the presence of large amounts of alkali sorbent material together with the activated carbon to enhance overall mercury removal. It is also specified that besides activated carbon, other powdery carbonaceous materials with some inherent adsorption activity, such as coal or coke, could also be used. However, these other carbonaceous materials do not normally possess sufficient inherent activity to be effective, even in combination with alkali reagent spray drying.
Activated carbon, the preferred sorbent for sorption of trace contaminants from fluid streams, is a predominantly amorphous solid having an extraordinarily large internal surface area (BET around 1000 m2/gm) and pore volume formed by activating a raw carbonaceous starting material such as coal, wood and biomass. The process of activation, which converts a raw carbonaceous starting material to a material that has a high adsorption capacity, is either a thermal or chemical activation process and can be equipment and energy intensive. Thermal activation typically involves various heating steps to pre-oxidize and devolatize the raw carbonaceous starting material followed by activation using steam, carbon dioxide or a mixture thereof at relatively high temperatures, sometimes greater than 800xc2x0 C. Chemical activation typically involves impregnating the raw carbonaceous starting material with a chemical activating agent and heating the mixture to a temperature between approximately 450-700xc2x0 C.
Both thermal and chemical processes are normally carried out in large rotary kilns with treatment times of several hours. The raw carbonaceous starting material is typically in the form of either briquettes, pellets or granules to prevent loss of the product through entrainment of fines during processing. Powdered activated carbon is then made by grinding the granular product. Therefore, the chemical and energy requirements to activate raw carbonaceous starting materials can be quite high, resulting in a relatively expensive activated carbon product. In addition, sorbent injection processes designed to remove vapor phase trace contaminants, such as mercury, found in low concentrations in gas streams with short residence times (approximately 1-10 seconds), require very large quantities of sorbent material. Therefore, the total cost for sorbent can be quite high.
In view of the foregoing, there exists a need for an improved method for removing vapor phase contaminants from a gas stream.
It is a general object of the present invention to provide a method for the removal of vapor phase contaminants from a gas stream.
A more specific object of the present invention is to provide a method for the removal of vapor phase air toxics, such as mercury, from the flue gas of a combustion process.
Another object of the invention is to provide a method for the removal of vapor phase air toxics, such as mercury, from the flue gas of a combustion process by reacting such air toxics with an activated material by injecting a raw carbonaceous starting material into the flue gas and activating it in-situ.
Another object of the invention is to provide a method as described above in which the source of raw carbonaceous starting material is relatively inexpensive, thereby avoiding the significant costs of pelletization, volatilization, activation and grinding associated with the production of commercially available activated carbons.
These objects are achieved by a method of, and apparatus for, activating a raw carbonaceous starting material in-situ in a gas stream, reacting the activated material with vapor phase contaminants and removing the activated material containing the vapor phase contaminants from the gas stream. The method includes the steps of injecting a raw carbonaceous starting material into a gas stream having an activation temperature at, or downstream of, the point of injection and a gas stream residence time sufficient to activate the raw carbonaceous starting material and then reacting this activated material with vapor phase contaminants, such as mercury. The activated material containing the vapor phase contaminants is then removed from the gas stream using a particulate collection device.
In another aspect of the invention, a gas stream source directs a gas stream to an activation region. The gas stream has an activation temperature at the activation region. A raw carbonaceous starting material is positioned at the activation region, such that the gas stream activates the raw carbonaceous starting material into an activated carbonaceous material. The activated carbonaceous material is then positioned in the exhaust stream of a combustion process to adsorb vapor phase contaminants.